1. Field of the Invention
The present invention relates to variable displacement piston pumps and more particularly to a variable displacement piston pump of the type which electrically controls its output pressure or both of its output pressure and output flow by a proportional electro-hydraulic control valve.
2. Description of the Prior Art
With hydraulically-controlled variable displacement piston pumps known in the art, particularly in the case of the axial piston type, it has been the practice so that in order to control the tilt angle of a swash plate to vary the output flow, the output pressure is introduced into a pressure chamber of a control piston serving as a variable element in response to the operation of a pressure control valve adapted to open when the setting pressure is reached and in this way a full cut-off condition is obtained. In this case, in the full cut-off condition the control piston pressure chamber is communicated with the low pressure side by the opening of a bleed hole preliminarily formed in a control piston sliding sleeve in response to the movement of the control piston and therefore there are disadvantages that to ensure matching between the bleed hole and the control piston length is extremely difficult due to variations in the processing tolerance thus tending to cause a vibration phenomenon of the control piston and that despite much efforts made to eliminate this phenomenon, generally such vibration preventing measures decrease the sharpness of the full cut-off characteristic with the resulting increase in the so-called pressure drooping.
On the other hand, methods have been known in which proportional electro-hydraulic control valves are used to control the output flow and output pressure of a variable displacement piston pump. In the case of the ordinary conventional method, the output pressure is controlled by using a proportional electro-hydraulic relief valve so as to perform a hydraulic-pressure feedback control to attain a setting pressure corresponding to its input current and the control of the output flow is effected by controlling the displacement of the output flow variable element of the pump by a separate proportional electro-hydraulic control valve.
With the variable displacement piston pump of the above type, due to the use of the separately provided proportional electro-hydraulic control valves for the control of the pressure and flow, respectively, not only the control valves but also the associated components such as current amplifiers for driving the control valves must be provided separately with the resulting unavoidable increase in the size of the system and the power consumption. Moreover, the conventional variable displacement piston pump involves varies output flow varying factors causing variations in the speed of actuators such as a hydraulic motor and cylinders with the result that if a variation in the slip of the pump driving electric motor due to a change in the load changes its rotation speed, this speed change results in a variation of the pump output flow, that if the volumetric efficiency of the pump is varied due for example to an increase in the load pressure, this also results in a variation of the output flow, that if a temperature change of the hydraulic working fluid causes a change in its viscosity, this also results in a variation of the output flow and so on and these factors are particularly noted in cases where accurate control is required.
Control systems of the type employing the conventional variable displacement piston pump in combination with hydraulic compensator valves for close control purposes have been proposed and the conventional control system of this type has been designed so that its pressure control section and flow control section are operable independently of each other. Thus, pressure compensation for variations in the load flow during the pressure control and flow compensation for variations in the load pressure during the flow control must be provided by the separate hydraulic compensator valves and also the pressure and flow control systems are in the form of open loops thus making it impossible for the control system to compensate for the effects due to a hysteresis of the solenoid means of the control valve, the viscosity of the working fluid, etc.